1. Field of the Invention
This invention relates to a method of manufacturing a semiconductor device by using a technique to directly bond two semiconductor substrates.
2. Description of the related art including information disclosed under .sctn..sctn.1.97-1.99.
In power semiconductor devices, with increase of the rated voltage, the specific resistance of the epitaxial wafer with an epitaxial layer formed by vapor growth process must be large. When the semiconductor substrate of high impurity concentration is used, it is very difficult to form a high resistance epitaxial layer in the substrate, because impurity from the substrate inevitably enters the epitaxial layer. For example, formation of an epitaxial wafer with n.sup.+ -n.sup.- junction whose n.sup.- layer resistance is 100.OMEGA..multidot.cm or more, for example, is very difficult.
In the conductivity-modulation-type MOS FET, for example, there are some instances where n.sup.+ -layer and n.sup.- layer are epitaxially grown on a p.sup.+ -substrate in a successive manner. In forming such an epitaxial wafer, the impurity compensation occurs at the interface between the p.sup.+ -n.sup.+ junction, so that desired junction characteristic can hardly be obtained. The same problem arises also in the case that the high impurity concentration layer is formed in a high impurity concentration layer whose conductivity type is opposite to that of the former, by a diffusion method.
As described above, when the conventional growth method or the diffusion method are used, the p-n junction characteristic obtained is always unsatisfactory.